Gravity meters suitable for use in boreholes and at other remote locations must meet requirements of ruggedness, accuracy, small size, and temperature stability.
A form of gravity meter heretofore used is described in U.S. Pat. No. 3,472,076 (1969) and employs an electrically conductive vibrating string having a mass fastened to its lower ends as a sensitive element. The vibrating string is suspended in the field of a magnet and vibrates at a natural frequency which changes as the attraction of gravity on the mass changes. The vibrating string gravity meter, such as described in the above patent, has excellent size and temperature characteristics but can be limited by characteristics of the vibrating string used as the sensitive element. Thus, for example, only one mode of oscillation in the string is desirable, but this is hard to achieve. Similarly, the string itself typically has temperature coefficient characteristics which can cause variations due to changes in ambient temperature. Likewise, over time the base frequency may change in steps, a phenomenon which may be the result of displacement slippage of, for example, the tungsten crystals of a vibrating string.